Implantable pulse generators (IPG) are devices that generate electrical stimuli to deliver to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system.
A spinal cord stimulation (SCS) system is a programmable implantable pulse generating system used to treat chronic pain by providing electrical stimulation pulses from an electrode array placed epidurally near a patient's spine. SCS systems consist of several components, including implantable and external components, surgical tools, and software.
Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an implantable pulse generator (IPG) or RF transmitter/receiver, insulated lead wires, and electrodes connected to the leads. The system delivers electrical pulses to the target nerves (e.g., dorsal column fibers within the spinal cord) through the electrodes implanted along the dura of the spinal cord. The leads exit the spinal cord and are tunneled around the torso of the patient to a subcutaneous pocket where the pulse generator/RF receiver is implanted.
Spinal cord and other stimulation systems are known in the art. For example, in U.S. Pat. No. 3,646,940, there is disclosed an implantable electronic stimulator that provides timed sequenced electrical impulses to a plurality of electrodes so that only one electrode has a voltage applied to it at any given time. Thus, the electrical stimuli provided by the apparatus taught in the '940 patent comprise sequential, or non-overlapping, stimuli.
In U.S. Pat. No. 3,724,467, an electrode implant is disclosed for the neural stimulation of the spinal cord. A relatively thin and flexible strip of physiologically inert plastic is provided with a plurality of electrodes formed thereon. The electrodes are connected by leads to a RF receiver, which is also implanted and controlled by an external controller. The implanted RF receiver has no power storage means, and must be coupled to the external RF transmitter/controller in order for neural stimulation to occur.
In U.S. Pat. No. 3,822,708, another type of electrical spinal cord stimulating device is shown. The device has five aligned electrodes that are positioned longitudinally on the spinal cord and transversely to the nerves entering the spinal cord. Current pulses applied to the electrodes are said to block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes are approximately 250 microseconds in width with a repetition rate of from 5 to 200 pulses per second. A patient-operable switch allows the patient to change which electrodes are activated, i.e., which electrodes receive the current stimulus, so that the area between the activated electrodes on the spinal cord can be adjusted, as required, to better block the pain.
The concern over how to power an IPG has also been addressed in various ways. For example, in U.S. Pat. No. 4,408,607, a type of capacitive energy source for an implanted medical device is disclosed. The power supply has a rechargeable capacitor that is used as the principal power source. The power source includes a lithium battery that is used as a second, or alternative, power source. The lithium battery replaces the capacitor as a power source during the charging period for the capacitor. This allows for continuous use of the device.
Regardless of past innovations, unique problems continue to be associated with providing power to IPGs, mainly because it is necessary to provide power to the device implanted below the skin. Commercial SCS systems are powered by one of three sources; a primary battery, a rechargeable battery, or an oscillating magnetic field (RF power). Since the device is subcutaneously implanted in a patient, either a bulky, external power source is used or an implanted power source must support device operation for a reasonable period of time in order to reduce further surgical trauma to the patient. RF powered devices suffer from poor patient acceptance due to the requirement of wearing an external device at all times.
If a battery is used as the energy source, it must have a large enough storage capacity to operate the device for a reasonable length of time. For low-power devices (less than 100 .u.W) such as cardiac pacemakers, a primary battery may operate for a reasonable length of time, often up to ten years. However, in many neural stimulation applications such as SCS, the power requirements are considerably greater due to higher stimulation rates, pulse widths, or stimulation thresholds. Powering these devices with conventional primary batteries would require considerably larger capacity batteries to operate them for a reasonable length of time (5 or more years), resulting in devices so large that they may be difficult to implant or, at the very least, reduce patient comfort. Therefore, in order to maintain a device size that is conducive to implantation, improved primary batteries with significantly higher energy densities are needed. However, given the state of the art in battery technology, the required energy density is not achievable at the present time.
For many neurostimulation applications, typical primary cells, such as non-rechargeable lithium thionyl chloride batteries are used, but they suffer from poor device lifetime causing the need for replacement surgeries. This means that during a twenty-five year period of time, a patient may be exposed to us to ten explant surgical procedures for the purpose of battery replacement. Not only is this inconvenient and costly, but as a surgical procedure, it necessarily involves some risk of infection or other complications.
One alternative power source is the secondary, or rechargeable battery, where the energy in these batteries can be replenished by transcutaneously recharging the batteries on a periodic basis. It is known in the art to use a rechargeable battery within an implant device. See, e.g., U.S. Pat. No. 4,082,097, entitled “Multimode Recharging System for Living Tissue Stimulators”, and U.S. Pat. No. 6,208,894, entitled “System of Implantable Devices for Monitoring and/or Affecting Body Parameters”, which patents are incorporated herein by reference. The devices and methods taught in this patent and application, however, comprise specialized devices, e.g., microstimulators, or relate to specific applications, e.g., cardiac pacing, which impose unique requirements not applicable to many IPG applications.
With the recent development of high capacity rechargeable power sources, rechargeable implantable pulse generators are possible. Due to the large energy density and the toxicity of the battery electrolyte, these batteries are typically contained within a hermetically sealed titanium case.
Super capacitors have been proposed as a replacement option for typical primary cell battery and RF power. One advantage of super capacitors is that they can be recharged approximately 500 times more than a rechargeable battery. This is a significant extension of lifespan over a rechargeable battery, and would substantially reduce the number of surgical procedures required for replacing batteries. Another advantage of super capacitors is their ability to be recharged very quickly.
Super capacitor technology, however, has not advanced enough to reduce super capacitors to a size that could be useful in an implanted IPG. The size of a super capacitor that is able to hold a charge for a reasonable amount of time—even for a twenty-four hour period—is too large to be integrated into an IPG that can be implanted, for example, in an individual's head.
In addition to the problem of applying an appropriate power source to run an IPG for a reasonable length of time, another problem exists with respect to powering IPGs. This problem relates to the loss of volatile memory, such as programming data, whenever the device is discharged. For example, U.S. Pat. No. 5,591,217 describes an implantable stimulator powered by large capacitors that can power the device for eight hours up to a few days. However, when the power source is drained, all programming data is lost and would need to be reloaded at each recharge cycle.